Static equipment including electronic components is now extensively used in telephone exchanges together with traditional electromechanical relay contacts to produce ringing signals (usually of 25 Hz) transmitted over subscriber lines to respective stations in response to digit-selection signals received by the exchange from calling subscribers. In the present state of the art, a ringing-signal generator normally comprises a pilot oscillator working into an amplification stage stabilized by negative feedback, the amplifier furnishing an output signal of the desired power characteristics. Two disadvantages of this arrangement are the low efficiency of such an amplifier and the large-size output transformer required to raise the voltage of the outgoing signal. Another conventional ringing-signal generator produces a stepped wave approximating a sinusoid, e.g. by connecting different taps of a transformer secondary in cyclic succession to an amplifier, this stepped wave being fed to a low-pass filter stage deriving a roughly sinusoidal voltage therefrom. Although such a device performs satisfactorily, the harmonics content of its output voltage is significant even with a high number of steps in the unfiltered waveform. Another shortcoming of the last-mentioned device is its relative inflexibility in the face of variations in load.